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Gravity’s secret signals

By Scott Faber

IMAGINE an event so violent that the Universe shudders when it occurs&colon; giant star exploding, one black hole devouring another, neutron stars colliding like billiard balls, even the birth of the Universe in the big bang. According to relativity theory, such cosmic quaking produces gravitational waves that propagate through the Universe like ripples on a pond. But these are no ordinary waves. Gravitational waves distort the very fabric of space, squeezing and stretching it as they pass. You might think that they would be easy to spot. They are not.

But now the hunt is on, as physicists and engineers in the US turn their minds to building instruments sensitive enough to detect gravitational waves. It is a daunting task. The squeezing and stretching they cause would change the length of a metre rule by about a millionth of the diameter of a proton. However, the size of the distortion depends on the size of the detector, so the trick is to make it extremely large. The instruments now being designed will have two giant arms. Each arm will consist of a vacuum-tight tunnel 4 kilometres long and over a metre wide with a laser at one end and mirrors at both ends. The two arms will be at right angles. The idea is that gravitational waves will stretch one and squeeze the other, while the laser measures the difference.

Cosmic xylophone

The project is called LIGO – the laser interferometer gravitational-wave observatory – and two detectors are planned. It is being carried out by physicists from the Massachusetts Institute of Technology (MIT) in Cambridge and Caltech …